(a-b)*(a-b)=10*10=100=aa-2ab bb
(b-c)*(b-c)=5*5=25=bb-2bc cc
(a-c)*(a-c)=15*15=225=aa-2ac cc
[aa-2ab bb] [bb-2bc cc] [aa-2ac cc]=100 25 225 =350
2aa 2bb 2cc-2ab-2bc-2ac=350
2*(aa bb cc-ab-bc-ca)=350
aa bb cc-ab-bc-ca=350/2=175